This invention generally relates to a novel method for increasing the surface area of metals, ceramics, and composites by creating laser induced micro-rough structures on the surface of the material. More particularly, the invention relates to the use of multiple, relatively unfocused, laser pulses to dramatically increase surface roughness by creating surface morphological structures on a scale measurable in microns. The invention also relates to the modified metal, ceramic and ceramic composite surfaces produced by this method.
In many applications for metals, ceramics and composites (ceramic-ceramic, ceramic-metal, metal-metal, ceramic-polymer and metal-polymer), maximizing the true surface area of the material is desirable. Without altering dimensions, this maximization is accomplished by toughening the surface, since a planar surface will have considerably less true surface area than a non-planar surface having, for example, numerous peaks and valleys. Conventional techniques for toughening ceramic and ceramic composite surfaces, especially those attempting to affect the surface on a micro scale, are not effective. Because of the hardness of ceramic material, machining, polishing, grinding or sand blasting the surface has does little to increase surface roughness, and much care must be taken during such processing due to the inherent brittleness of many ceramics. Chemical and thermal etching likewise produce little increase in surface roughness. These processes mainly affect grain boundaries and have little effect within the grains themselves. Increasing the porosity of the ceramic during initial processing can increase true surface area, but the increase in porosity is accompanied by a decrease in strength of the final ceramic. Maximizing surface area or roughness on metal surfaces on a micro scale is also difficult, since melt flow characteristics often affect the final morphology.
Examples of applications where maximum surface area of a ceramic or metal material is desirable include ceramic catalysts and catalyst supports, bio-ceramic bone and dental implants, battery electrodes, structural composite materials where bonding strength is crucial, heat exchange and transfer ceramics, sensors, anti-reflective ceramics, particle filtration, fuel cells and applications where improved adhesion is required.
A novel and unique method has been developed whereby the inherent surface area of metals, ceramics and composites can be increased up to two orders of magnitude using multiple laser pulses. The method is independent of initial processing and is performed on the material surfaces after production. The method affects only the outermost surface of the materials to a depth of up to 200 microns, or as little as 1/4 microns. Surface roughness and area is increased by the formation of periodic or semi-periodic micro-scale morphological structures in the nature of cones, peaks or columns having thicknesses of from 1/4 to 100 microns and heights of from 1/4 to 200 microns, with corresponding valleys or grooves with widths of from 1/4 to 300 microns. The structures can be oriented in particular directions and at particular angles by controlling the incidence angle of the laser beam.
The technique of using laser pulses to affect surfaces is known. Typically, the use of lasers falls into two categories, one where the laser is used on a macro scale to perform cutting or large scale surface removal on metals or plastics, and the second where the lasers are used to affect the surfaces on a smaller scale. The latter technique involves using a relatively focused laser beam to create surface morphologies by melt flow or shock waves, i.e., the beam melts the surface of the material to create craters by movement of the liquid. Only a small number of pulses can be utilized on a given spot and the technique does not produce periodic or semi-periodic structures.
It is an object of this invention to provide a novel and unique method for increasing the surface area of metal, ceramic and composite materials though the use of multiple laser pulses, the surface morphology being affected on a micro scale by the creation of surface morphological structures on the magnitude of from 1/4 to several hundred microns. The surface area of a treated metal, ceramic or composite is increased by up to a factor of 100 and the strength of the material is unaffected by the methodology.
It is a further object to provide such a method suitable for any metal, ceramic or composite and where the magnitude of the micro-roughness is controllable by varying parameters of the laser, such as number and duration of pulses, wavelength, energy density, angle of incidence and treatment environment, which in turn are dependent on the particular metal, ceramic or composite being treated.
It is a further object to provide novel metal, ceramic or composite surfaces of increased surface roughness produced by the method described above, the surfaces having periodic or semi-periodic morphological structures on the magnitude of from 1/4 to several hundred microns.